Process for forming honeycomb extrusion die

ABSTRACT

Honeycomb extrusion dies for the extrusion of honeycomb ceramics of high cell density and reduced cell wall thickness are machined from fully consolidated powder-formed (P/M) stainless steels, providing dies with reduced feed hole roughness, improved feed hole straightness, and superior discharge slot finish, with the result that significantly enhanced extrusion performance and higher quality honeycomb extrusions are realized.

This is a division of application Ser. No. 08/337,252, filed Nov. 10,1994.

BACKGROUND OF THE INVENTION

The present invention relates to extrusion dies for forming thin-walledhoneycomb structures from extrudable materials. More particularly, theinvention relates to extrusion dies offering improved utility for theextrusion of thin-walled high-cell-density ceramic and metallichoneycombs.

In the manufacture of ceramic honeycombs, particulate mineral batchmaterials are dispersed in an appropriate vehicle to form a plasticizedpowder batch, and the batch is forced through a honeycomb die to provideextruded green bodies of complex honeycomb shape which are then driedand fired. Thin-walled ceramic honeycomb structures thus produceddisplay utility in a variety of applications. For example, suchstructures are used as substrates for the support of catalysts inautomotive exhaust gas treatment systems, as well as for other catalystcarriers, filter bodies, and thermal regenerators or heat exchangers.Metallic honeycombs of similar configuration have been used as gasheaters.

As the art of honeycomb extrusion has advanced, honeycombs with finerand finer cell structures and thinner cell walls have been developed.The production of these finer honeycombs requires that extrusion dieswith finer structure be used. Dies used for the extrusion of ceramichoneycombs are typically machined metal plates or blocks having shallow,crisscrossing and interconnecting discharge slots on the downstream oroutlet face of the die from which the plasticized batch emerges andthrough which, during emergence of the batch, the webs or sidewalls ofthe cells of the honeycomb structure are formed. To supply the batchmaterial to the discharge slots, feed holes communicating with the slotsare provided in the opposite or inlet face of the die.

To provide finer honeycombs with higher cell densities (more cells perunit area) and thinner cell walls, the discharge slots and feedholesmust of course be formed closer together and be smaller in size. Foradvanced ceramic honeycomb products, the objectives are to achieve celldensities exceeding 600 cells/in² and cell walls below about 200 μm inthickness.

To achieve these dimensions, slot and feedhole machining to very closetolerances is required. Meeting such tolerances has required the use ofnon-traditional machining processes such as electrochemical machining(ECM) and wire electrical discharge machining (EDM). EDM is thepreferred process for generating the discharge slots and ECM istypically used for producing the precise arrays of small feedholesneeded to supply the discharge slots.

In the prior art, extrusion dies for the manufacture of ceramichoneycombs have been formed of tool steels or stainless steels.Stainless steels are harder to machine but offer significant advantagesfor honeycomb die fabrication because they offer a corrosion resistantmedium which can withstand relatively high stresses and attack byaqueous media. In addition, many stainless steels can be wear-coatedwith hard surfacing materials such as carbides and nitrides. Suchcoatings significantly enhance the ability of the die to resist wearfrom the abrasive ceramic powder batch materials extruded therethrough.

The difficulty of shaping these very hard stainless steel materials hasled to the suggestion of alternative extrusion die manufacturingstrategies, particularly where complex feedhole shapes are desired. ThusU.S. Pat. No. 5,308,556, for example, discloses a method of forming anextrusion die from a powder, wherein a powder preform for the die,typically of ceramic composition but optionally of metal, is at leastpartially machined while in a porous and unconsolidated (green orchalk-hard) state. The shaping of consolidated die blank materials isalso mentioned, although specific materials useful in that procedure arenot actually described or discussed.

Unfortunately powder-formed dies made as described in the above patenthave not yet been proven for use in the production of high-cell-density,thin-walled honeycomb structures. One problem with the describedapproach is the difficulty of maintaining high dimensional precision infeedholes and/or discharge slots made in unconsolidated materials duringthe high-shrinkage process of consolidating them to useful densities.Thus the materials of choice for the fabrication of advanced honeycombextrusion dies are still wrought stainless steels and tool steels.

Descriptions of the use of EDM and ECM for the fabrication of stainlesssteel extrusion dies are found in the patent literature. U.S. Pat. Nos.5,320,721 and 5,322,599, for example, describe the application of ECMprocesses to the machining of die feedholes, while U.S. Pat. No.4,527,035 documents the application of wire EDM to the machining ofdischarge slots in the outlet faces of the dies.

In principle, the finer hole and slot dimensions needed for advancedhoneycombs can be reached with ECM and EDM machining techniques. Inpractice, however, the resulting dies do not demonstrate the expectedextrusion performance. Forming defects including missing webs(interruptions in the formation of the cell walls of the honeycomb) andswollen webs (wall segments of excessive thickness), are often observed,as is unacceptable twisting or turning ("bowing") of the extrudedmaterial as it exits the extrusion die. In general, these defects areusually attributed to defects in design or finish of the extrusion dies.

It would be desirable to develop a die or die machining procedure formaking dies for the extrusion of very fine honeycomb structures whichwould permit the extrusion of honeycombs with thinner walls and/orhigher cell counts at yields as high or higher than present honeycombextrusion processes.

It would also be desirable to develop a die which would permit theextrusion of less advanced ceramic honeycombs at higher yields and inhigher quality. Yet, any material selected for this application wouldhave to be sufficiently strong to handle the relatively large extrusionpressures required for fine honeycomb extrusion, and sufficientlydurable and wear resistant to resist the abrasive effects of presentlyused ceramic powder batches.

SUMMARY OF THE INVENTION

The present invention is based on the finding that fully consolidatedpowder-formed stainless steels, that is stainless steels formed fromsteel powders which have been consolidated to a dense, substantiallynon-porous state, provide much better die blank materials for thefabrication of honeycomb extrusion dies than do conventional or wroughtstainless steels of similar composition. Die blanks formed from thesepowder-formed or so-called P/M (powder metallurgy) stainless steels, ifof appropriate composition and density, have been found to yield diesoffering significant improvements in surface finish and extrusionperformance. These improvements are evident both from a study of thedies themselves and from the quality of the extruded honeycombs.Improvements are seen not only for advanced honeycomb designs of highcell count and fine wall structure, but for current honeycomb designs aswell.

In a first aspect then, the present invention comprises an improvementin a method for making a honeycomb extrusion die from stainless steel.As is conventional, that method includes the steps of forming feedholesand discharge slots in the inlet and outlet faces of the die. However,in accordance with the invention, the stainless steel die blank selectedfor fabrication of the die is a blank made of a powder-formed stainlesssteel which has been consolidated to a dense non-porous state.

Powder-formed or P/M stainless steels are well known in themetallurgical arts, but have been used mainly for the forming of steelparts of complex configuration. Using P/M technology, such parts can beformed to near net shape by pressing metal powders into compacts andthen consolidating. The aim, of course, is to avoid as much as possiblethe need for machining.

Similarly, porosity is a desired attribute of some P/M metals, withbending strength being of relatively little importance. Porosity is ofparticular interest for applications such as bearings, whereinfiltration of the metal matrix with lubricous materials is desired.

The present invention does not seek to avoid the machining of P/Mstainless steel, but instead to select such a steel which issufficiently strong, homogeneous and dense to provide a durable dieblank material which can be machined to very high dimensional tolerancesand a surface finish substantially free of defects on a micron scale.Likewise, porosity in the steel is to be avoided, both for reasons ofsurface finish and because die steels must exhibit high strength inorder to resist extrusion pressures without deformation.

Powder-formed stainless steels have now been identified which meet allof these requirements, and which in addition are sufficiently improvedin homogeneity and free of inclusions and other crystalline matrixdefects as to provide a large and unanticipated improvement inmachinability. The invention thus further includes a honeycomb extrusiondie formed of such a material, wherein the die blank from which the diehas been fabricated is a fully consolidated powder-formed stainlesssteel blank substantially free of microstructural porosity andintermetallic inclusions. Such a die offers all of the advantages ofconventional stainless steel dies in terms of strength, corrosionresistance and wear coatability, but in addition demonstrates superiorextrusion performance in terms of product quality and process stability.

DESCRIPTION OF THE DRAWINGS

The invention may be further understood by reference to the drawings,wherein:

FIG. 1 is an electron photomicrograph of a section of a wrought steeldie blank material of the prior art; and

FIG. 2 is an electron photomicrograph of a section of a P/M steel dieblank material provided in accordance with the invention.

DETAILED DESCRIPTION

While the variations in extrusion efficiency observed in attempting toextrude fine honeycomb structures in the prior art were generallyattributed to variations in the extrusions dies, the factors whichdifferentiated dies exhibiting good extrusion behavior from diesexhibiting poor behavior were not well understood. In the course ofdeveloping dies with finer feedhole and discharge slot configurations,these variations assumed much greater importance and had to beaddressed.

In the ECM process typically used for drilling the very fine feedholearrays in stainless steel die blanks, a frequent problem had been bentdrilling tubes. Such bending produced angling or "spearing" of thefeedhole away from the intended line of drilling, with the result thatsome of the feedholes failed to properly intersect with the dischargeslots. In the machining of discharge slots by the EDM processes, the useof finer wires to achieve finer slot patterns resulted in a higherincidence of wire breakage and uneven slot widths in the final slotpattern.

Analysis of die blanks exhibiting significant machining problems did notidentify any significant departures from specifications for targetedchemical composition and physical properties. In fact, stainless steelsof the same AISI type but from different lots, and even from differentlocations on a single length of bar stock from a single lot, producedwidely varying results when subjected to drilling and slotting.

One variable which did associate with machining problems, however, wasmachined surface roughness. Analysis of a large number of samples fromdifferent bar stock locations and different lots of steel of the sametype indicated that there was a strong correlation between feedholedrilling defects and the surface roughness of machined surfaces of thefeedholes. In one study, conducted on AISI Type 450 stainless steel froma commercial source, machined hole surface roughness values (Ra) were35-40 microinches in some bar sections and 60-65 microinches in othersections. Dies made from the blanks with higher machined surfaceroughness were found much more likely to produce speared feedholesand/or extrusions which bowed away from or twisted about the line ofextrusion.

Microscopic examinations of steel samples undertaken in an effort tomore completely understand these effects revealed that the steels withhigher surface roughness had higher levels of intermetallic inclusionsin the steel matrix. These inclusions appeared in many cases to havebeen rolled out into long "stringers" during the steel forging process,increasing the possibility that the inclusions would intersect afeedhole or a slot.

While not yet fully understood, these inclusions appear to be rich inniobium, perhaps consisting largely of niobium carbide. Such inclusionswould not be readily dissolved or eroded during electrochemical drillingprocesses, and could therefore be at least partly responsible foreffects such as drilling tube deflections and higher surface roughnessin selected feedholes, as well as discharge slot irregularities and wirebreakage during the wire EDM slotting process.

The above findings suggested that the majority of the feedhole anddischarge slot defects observed in dies fabricated from stainless steelsare attributable not simply to wire EDM and ECM machining processinglimitations and/or variability, but at least in part to microstructuraldefects in the stainless steels used to form the dies. Such defectsalthough small, could promote machining variations which could berelatively large on the scale of the feedhole and discharge slotdimensions being required.

Regardless of theory, in accordance with the present invention thereduction or substantial elimination of the above-described machiningdefects is achieved through the substitution of fully consolidatedpowder-formed stainless steel for the wrought steel stock conventionallyused for the die blanks. These powder-formed consolidated stainlesssteels are apparently sufficiently improved in homogeneity andsufficiently free of inclusions and other crystalline matrix defects asto provide a large improvement in the smoothness and uniformity of thefeedholes and discharge slots in these dies.

A comparison of the microstructure of wrought and powder-formedstainless steel samples useful for the fabrication of honeycombextrusion dies is provided in FIGS. 1 and 2 of the drawings. FIG. 1 isan electron photomicrograph of a conventional or wrought steel sample,taken at a magnification of 400×, wherein the black bar represents adimension of ten microns. The sample shown is a wrought AISI Type 450stainless steel, polished with one micron diamond abrasive and etched ina 4% Picral solution with HCL to reveal the microstructure of thesample.

As is evident from this photomicrograph, the steel of FIG. 1 includes asubstantial number of intermetallic inclusions in the steel matrix, withseveral concentrations or clusters of such inclusions being indicated bythe arrows. As suggested above, these inclusions can form elongated orline defects in the structure, called "stringers", which are now thoughtto interfere with chemical and electrical machining processes.

FIG. 2 of the drawing is a similar electron photomicrograph taken of apowder-formed AISI Type 422 stainless steel. Again, the surface of thesample is shown at a magnification of 400× after polishing and etchingwith 4% Picral in HCL. In contrast to the microstructure of FIG. 1, theetched microstructure in this photomicrograph is substantially free ofthe intermetallic inclusions seen in the wrought sample of the samecomposition. In addition, though formed from a powder, it issubstantially free of microstructural porosity, even at magnificationsup to 100×. The use of a fully consolidated P/M stainless steel such asshown in FIG. 2 for the fabrication of a honeycomb extrusion die isdescribed in the following illustrative example.

EXAMPLE

A plate formed of P/M stainless steel is selected for use as anextrusion die blank. The steel employed is an AISI Type 422 stainlesssteel, commercially available in bar form as Carpenter 636 stainlesssteel from Carpenter Technology Corporation, Reading, Pa. This steel ismanufactured by atomization of a molten stainless steel stream in a highvelocity gas jet to a fine powder, the fine (325 mesh) powder then beingconsolidated by hot isostatic pressing into large steel billets. Thebillets are then cogged and rolled to provide steel bar and plate stock.

To fabricate a die from the steel plate thus provided, an array of finefeed holes is first drilled into one surface of the plate by theelectrochemical machining (ECM) process. The drilling process used isthe same process as used conventionally for drilling wrought Type 422stainless steel, and the result is the formation of an array of finefeed holes in the drilled surface of the plate.

After the plate has been drilled, it is turned over and the surfaceopposite the feed holes is slotted to provide a discharge slot array inthe opposite surface connecting with the previously drilled feed holearray. The discharge slots are machined into the steel by wireelectrical discharge machining (EDM). Again, the EDM process used is thesame as is used to machine wrought Type 422 stainless steel for thispurpose.

Both the feed hole drilling and discharge slot machining of this P/Mblank are accomplished with relative ease. The incidence of rough orspeared feed holes and EDM wire breaks is significantly reduced whencompared with machining results for the same processes as applied towrought stainless steels.

For example, the experience with wrought stainless steel, most typicallyType 450 stainless steel as shown in FIG. 1 of the drawing, is thatdefective feed holes, including holes with excessive roughness and/orpoor intersection with the discharge slots, typically comprise 20% to30% of the holes drilled by the ECM process. In contrast, recentexperience with fully consolidated P/M stainless steels, such as the P/MType 422 steel shown in FIG. 2 of the drawing, is that excessivelyrough, speared or poorly intersecting feedholes are virtually eliminatedduring the ECM drilling of the P/M materials.

Similarly, EDM wire breaks encountered when slotting wrought stainlesssteels, using for example 5-mil and 6-mil wire of the kind typicallyused for slotting honeycomb extrusion dies, generally average at least20 breaks during the slotting of die blanks of the size currently usedfor the production of automotive catalyst converter substrates. Thiscontrasts markedly with recent data collected on the slotting of similardie blanks of powder-formed stainless steel as in the Example. In lattercase fewer than two wire breaks are typically seen during the EDMslotting of P/M blanks of the same size.

The effects of these improvements in die quality on the extrusion ofceramic honeycombs is significant. Most importantly, the incidence of"bow" as a new die is introduced into the extrusion process can besignificantly reduced by employing P/M stainless steel as the die blankmaterial. In fact, the percentage of new dies requiring polishing orhoning to correct bow problems can be reduced by a factor of at least 4by substituting powder-formed stainless steel blanks for wroughtstainless steel blanks in the die fabrication process.

At present, the particular composition of the powder-formed stainlesssteel selected for use as a die blank material in accordance with theinvention is not considered to be critical. Thus, the steel typeselected from among the available P/M sources may be chosen primarily onthe basis of factors such as strength, wear coatability, hardness, andthe like.

The presently preferred powder-formed stainless steels for extrusion diefabrication are the chromium-containing ferritic or martensitic 400series stainless steels, specific examples of such steels being Types450 and 422 steels. At present, the particularly preferred steel is P/MType 422 stainless steel.

Depending on the particular material being extruded and/or differingrequirements for wear coating or other die hardening procedures, otherstainless steels could alternatively be employed. Examples of such otherof steels, considered excellent candidates for extrusion die use ifsourced from powdered steel, are the chromium- and nickel-containingaustenitic or precipitation-hardenable steels. An example of such asteel is Type 17-4PH (AISI Type 630) precipitation-hardenable steel.Even durable non-steel alloys, including nickel alloys such as certainof the Inconel™ alloys, could constitute excellent die blank materialsif sourced from powders and consolidated to dense void-free blanks.

Key requirements of any of these candidate extrusion die materials arethat they be available in a fully consolidated form which issubstantially free of microstructural porosity, resistant to corrosion,and of sufficient strength to withstand the stresses of the extrusionprocess for which they are intended. Additionally important for someapplications are the thermal processing characteristics of the metal,since thermal stability improves wear-coating compatibility, and thusthe suitability of the die material for ceramic batch extrusionapplications.

I claim:
 1. In the process for forming a honeycomb extrusion die bymachining intersecting discharge slots and connecting feedholes inopposing surfaces of a steel die blank, the improvement wherein:thesteel die blank is formed of a fully consolidated powder-formed,stainless steel which is substantially free of intermetallic inclusionsand microstructural porosity.
 2. A process in accordance with claim 1wherein the feedholes are formed by electrochemical machining.
 3. Aprocess in accordance with claim 2 wherein the discharge slots areformed by wire electrical discharge machining.
 4. A process inaccordance with claim 1 wherein the steel die blank is formed of apowder-formed stainless steel selected from the group consisting offerritic and martensitic stainless steels.
 5. A process in accordancewith claim 1 wherein the steel die blank is formed of a powder-formedstainless steel selected from the group consisting of austenitic andprecipitation hardenable stainless steels.
 6. A process in accordancewith claim 1 wherein the steel die blank is formed of P/M Type 422stainless steel.